U. S. Pat. No. 3,881,458 Roozenbeek et al assigned to the assignee of this application, is directed to an ignition system in which a main current supply transistor is connected in series with the ignition coil to control current flow therethrough. A stabilized voltage circuit, including a Zener diode, is provided, connected to the source of power for the transistorized ignition circuit, typically an automotive battery. A pulse source provides pulses to command ignition events; this may be a breaker-controlled pulse source, or an inductively or otherwise controlled pulse source. A controlled transistor is connected to the pulse source and, in turn, controls through an intermediate coupling transistor the main ignition current supply transistor. The intermediate coupling transistor is so arranged that, when the main current supply transistor is conductive, it itself is blocked, so that effectively full battery or operating voltage can be applied to the control terminal of the main ignition current supply transistor when it is conductive.
It has been observed that occasionally the main operating or ignition switch of the motor vehicle may be broken just at the time when the main power supply transistor is conductive and while a piston of the engine is in a position which, should an explosion occur, might damage the engine, or components thereof. If, at this moment, the ignition switch is turned off, the main power supply transistor will block, triggering an ignition event which may cause this damage.